13 inches is very short for the length of a garden. You haven’t said what units the area is measured in. Is it 6 square feet? Square inches? If 13 inches was the length and 6 sq in the area, the width would be 6/13 of an inch! If the area was 6 sq ft, then the width would be 864/13=66.5 inches approximately, which makes the garden much wider than the length. The perimeter is twice the sum of length and width.
Let’s assume the garden is 13 in longer than it is wide and that the area is 6 sq ft=6×144=864 sq in.
If the length is 13 in more than the width, w, then the area is w(w+13)=864.
So w²+13w=864 or w²+13w-864=0. This gives us w=23.6 in approx and length=36.6 in.
The perimeter is 2(width+length)=120.4 in.
If this is not the answer, check the wording of the question again and resubmit. Make sure you specify all the units correctly (inches, feet, square inches, square feet, etc).